Before Facebook, there weren't any online communities where people revealed their true identities, Zuckerberg explains.

When he was creating Facebook, he needed a way to prove that users were, in fact, students. Facebook used school email addresses to vouch for the students and show they were who they claimed to be. It also prevented fake accounts from being created, because each student typically had one email account per school.

"One of the things that people don't think about today was, early on, we wanted to establish this culture of real identity on the service," Zuckerberg says. "I assume probably around 2000 all schools started issuing email addresses, but that was really the critical thing that made it so we could get started. But it was this counter-intuitive thing that not many other services were using."